In medical imaging, biological tissue properties are used to produce image contrasts. For example the electrical conductivity differs for different types of tissue. Therefore a conductivity quantity characterizing the conductivity distribution throughout at least a portion of a human body is significant for distinguishing different tissues of a human body. For instance, said conductivity quantity can be used to distinguish between tumours and healthy tissue or between necrotic and vital tissue after a myocardial infarction. Said conductivity quantity can also be used to support the characterization of brain tissue in connection with stroke or cerebral haemorrhage.
Numerous methods for determining electromagnetic quantities are known. For example a method called “Electric Impedance Tomography” (EIT) is used for conducting conductivity measurements and hence for determining a conductivity quantity. With this method numerous conducting electrodes are attached to the skin of a person to be examined and an electric current is applied across the electrodes. A great disadvantage of this method is the enormous amount of time needed for attaching the electrodes. There is also the fact that the spatial resolution is not very high.
In WO 2007/017779 A2 a method called “Electromagnetic Properties Tomography” (EPT) using a MRI system or MRI scanner is described. With this method an electrical permittivity distribution and/or an electrical conductivity distribution throughout a patient's body can be determined. With this method an excitation electro-magnetic field is applied to excite spins of an object. Magnetic resonance signals from the excited object are acquired. A magnetic induction field strength distribution is derived from the acquired magnetic resonance signals. Furthermore, an electric field strength distribution associated with the excitation electro-magnetic field is computed using the magnetic field and the Maxwell equations. The electrical permittivity distribution and/or the electrical conductivity distribution are computed from the electric field strength distribution and the magnetic induction field strength distribution. Although the amount of time needed for doing preparations before a measurement can be conducted is comparatively small, the use of this method is restricted because of the following disadvantages: Firstly, the frequency of the excitation electro-magnetic field is fixed to the so-called Larmor frequency of the MR system involved, with this frequency being significantly higher than the frequencies required for the majority of corresponding investigations. Secondly, as it is impossible to rotate the excitation electro-magnetic field with a MR system, an anisotropy of the electric conductivity can be investigated only for very few, partially “rotatable” body parts like hands, feet, and head. Thirdly, a MR scanner is a rather expensive imaging modality, particularly since for EPT it is “just” used to generate and measure magnetic fields.
“Magnetic Particle Imaging” (MPI) is an emerging medical imaging modality. The first versions of MPI were two-dimensional in that they produced two-dimensional images. Future versions will be three-dimensional (3D). A time-dependent, or 4D, image of a non-static object can be created by combining a temporal sequence of 3D images to a movie, provided the object does not significantly change during the data acquisition for a single 3D image.
MPI is a reconstructive imaging method, like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). Accordingly, a MP image of an object's volume of interest is generated in two steps. The first step, referred to as data acquisition, is performed using a MPI scanner. The MPI scanner has means to generate a static magnetic gradient field, called “selection field”, which has a single field free point (FFP) at the isocenter of the scanner. In addition, the scanner has means to generate a time-dependent, spatially nearly homogeneous magnetic field. Actually, this field is obtained by superimposing a rapidly changing field with a small amplitude, called “drive field”, and a slowly varying field with a large amplitude, called “focus field”. By adding the time-dependent drive and focus fields to the static selection field, the FFP may be moved along a predetermined FFP trajectory throughout a volume of scanning surrounding the isocenter. The scanner also has an arrangement of one or more, e.g. three, receive coils and can record any voltages induced in these coils. For the data acquisition, the object to be imaged is placed in the scanner such that the object's volume of interest is enclosed by the scanner's field of view, which is a subset of the volume of scanning.
The object must contain magnetic nanoparticles; if the object is an animal or a patient, a contrast agent containing such particles is administered to the animal or patient prior to the scan. During the data acquisition, the MPI scanner steers the FFP along a deliberately chosen trajectory that traces out the volume of scanning, or at least the field of view. The magnetic nanoparticles within the object experience a changing magnetic field and respond by changing their magnetization. The changing magnetization of the nanoparticles induces a time dependent voltage in each of the receive coils. This voltage is sampled in a receiver associated with the receive coil. The samples output by the receivers are recorded and constitute the acquired data. The parameters that control the details of the data acquisition make up the scan protocol.
In the second step of the image generation, referred to as image reconstruction, the image is computed, or reconstructed, from the data acquired in the first step. The image is a discrete 3D array of data that represents a sampled approximation to the position-dependent concentration of the magnetic nanoparticles in the field of view. The reconstruction is generally performed by a computer, which executes a suitable computer program. Computer and computer program realize a reconstruction algorithm. The reconstruction algorithm is based on a mathematical model of the data acquisition. As with all reconstructive imaging methods, this model is an integral operator that acts on the acquired data; the reconstruction algorithm tries to undo, to the extent possible, the action of the model.
Such an MPI apparatus and method have the advantage that they can be used to examine arbitrary examination objects—e.g. human bodies—in a non-destructive manner and without causing any damage and with a high spatial resolution, both close to the surface and remote from the surface of the examination object. Such an arrangement and method are generally known and are first described in DE 101 51 778 A1 and in Gleich, B. and Weizenecker, J. (2005), “Tomographic imaging using the nonlinear response of magnetic particles” in nature, vol. 435, pp. 1214-1217. The arrangement and method for magnetic particle imaging (MPI) described in that publication take advantage of the non-linear magnetization curve of small magnetic particles. As yet MPI apparatuses and methods are not adapted for determining electromagnetic quantities.